1. Field of the Invention
The present invention relates to a wrench with a maximum operational torque to prevent damage to the object secured by a fastener driven by the wrench.
2. Description of the Related Art
Fasteners such as screws, bolts, etc are widely used to secure objects in place. The objects might be damaged when overtightened. U.S. Pat. No. 6,666,117 to the applicant of the present application discloses a wrench comprising a rod, a retainer securely engaged with the rod to move therewith, a casing for accommodating the retainer, a ball, and an elastic element. The ball is biased by the elastic element to engage with a retaining section of the casing to thereby retain the retainer in place. When a rotational force applied to the casing is smaller than the engaging force between the retaining section of the casing and the ball, the retainer and the rod are turned to thereby turn the fastener for tightening purposes. When the rotational force applied to the casing is greater than the engaging force between the retaining section of the casing and the ball, the retainer and the rod slide and the fastener is not turned. Thus, the user will notice the sliding motion of the casing and be aware of tightening of the fastener, thereby preventing damage to the object secured by the fastener. The engaging force, which largely depends on the elastic coefficient of the elastic element, determines the fixed maximum operational torque. The fixed maximum operational torque can be altered by selecting elastic elements of different elastic coefficients.
However, the engaging force provided by the elastic element is affected by the tolerance in the diameter of the elastic element, the overall length of the elastic element, the period of time and/or temperature of heat treatment, etc. Namely, the engaging force could not be obtained to the desired precise extent. Minor adjustment of the engaging force or maximum operational torque is not allowed in this wrench as well as other conventional wrenches.